The invention is directed to a circuit arrangement for automatic gain control AGC. Automatic gain control is used in many types of equipment, for example in radio receivers or CD players to regulate the maximum output level at a constant level independently of fluctuations in the receiver field strength. In a CD player, or a magneto-optical recording and playback device, a laser beam illuminates an optical recording medium which reflects the laser beam to a photodetector having a plurality of photodiodes. In addition to a data signal, a focusing error signal used in focusing a regulation circuit, and a tracking error signal used in a tracking regulation circuit, are generated from the output signals of the photodiodes. Changes in the laser performance, or in the reflectance capability of the optical recording medium result in changes in the loop amplification of the focus and the tracking regulation circuits.
In an optical reproduction device, described in U.S. Pat. No. 4,835,755, the analog focusing error signal, the analog tracking error signal and the analog speed regulation signal which regulates the r.p.m. of a disk drive motor, are converted to digital signals by several analog-to-digital converters. The digital signals are evaluated in a microprocessor. The microprocessor provides digital regulating signals, which are reconverted to analog signals. The analog regulating signals regulate the actuators of the focus and tracking regulation circuits and the rpm of the disk drive motor.
A further loop amplification circuit is shown in a brochure from Analog Devices which describes an IC component AD 533. This IC device includes an analog divider circuit in which the focusing error signal and the tracking error signal are divided by a value proportional to the total light energy arriving on the photodetector. The focusing error signal and the tracking error signal are, therefore, standardized with respect to the total incoming light energy. However, this solution has two significant disadvantages, the analog divider circuit is relatively expensive and a substantial amount of equalization is required. The invention provides an automatic gain control circuit which is easily manufactured and which requires no equalization.